Weed issues continue to challenge California no-till farming

No-till farming is about improving the resource base and remaining competitive.
With no-till, 90 percent of weed seeds are in the top 2 inches of soil. With conventional tillage, 10 percent to 12 percent are in the top 2 inches.
Costs for weed control in conventional and conservation systems pencil out about the same – 20 percent to 30 percent of production costs.

Dennis Pollock | Jul 20, 2011

Jeff Mitchell, cropping system specialist with the University of California at Davis, did a lot of looking over his shoulder at a cotton-tomato no-till field day in Five Points.

But while talking about global competition in precision agriculture and the need to be competitive, he also had one eye on the future and the need to tweak conservation tillage systems he has researched for more than a decade at the University of California Westside Research and Extension Center in Five Points and elsewhere.

“This is about improving the resource base and remaining competitive,” Mitchell said, citing efforts in Brazil, where 65 percent of processing tomatoes are grown with no-till systems, and Turkey, where silage wheat is followed by the no-till planting of transplant tomatoes.

Mitchell concedes it’s been a challenge to show California cotton can benefit from the systems, but he is encouraged by news in a recent Western Farm Pressstory[2] on cover crops for Arizona cotton production. And he believes systems can be improved as tweaking in California continues with moves to subsurface drip and overhead linear irrigation that could use less water and be more efficient than furrow irrigation.

“Are you likely to be doing the same thing you’re doing today in 20 years?” Mitchell asked growers at the field day.

At the Five Points center, cover crops – including triticale, which is also used in the Arizona systems – are used as mulch. Mitchell said benefits include increasing carbon and organic matter in the top foot of the soil. Advantages of conservation tillage include reducing trips across the field, cutting labor and fuel costs and reducing air pollution.

However, challenges remain, and speakers at the field day enumerated some of them, starting with weed management.

With no-till systems, weed banks in the top 2 inches of soil can increase significantly and the types of weeds can change to more challenging ones, said Anil Shrestha, professor of plant sciences at California State University, Fresno.

With tillage, weed seeds are likely to be buried at greater depths, Shrestha explained.

With no-till, 90 percent of weed seeds are in the top 2 inches of soil. With conventional tillage, 10 percent to 12 percent are in the top 2 inches.

“But by the fifth year, things start to stabilize,” Shrestha said. “Still, there should be zero weed tolerance for no-till systems.”

He said there is likely going to be a species shift for no-till, with nutsedge and bindweed, for example, becoming more common. Over time, there may be more weed densities in standard than no-till systems, he said, but no-till systems with no cover crops had the lowest weed densities, suggesting it’s best to try to get a weed-free cover crop.

Shrestha said that he and other researchers sampled the seed banks in fields at the Five Points station shortly after a heavy rain and found that water infiltration was better in no-till systems “and it was very difficult to sample in no-till without hitting an earthworm. There were none in the conventional plots.”

Costs for weed control in conventional and conservation systems “pencil out about the same – 20 to 30 percent of production costs,” said Kurt Hembree, UCCE weed management farm in Fresno County.

Battling weeds early

But he warned it is important to keep neighboring weeds out of the seed bank early, pointing out pigweed and horseweed bordering the field and likely to seed it if left unattended.

He said weeds in the sunflower family thrive in no-till environments. “They drift in on the wind and like shallow, undisturbed conditions.” Those pesky seeds can come from plants that include fleabane and mare’s tail.

Hembree sounded a special warning against herbicide resistant Palmer amaranth. “Fleabane and this other stuff are nothing compared to that,” he said.

When making a transition to no-till, he said, it might be wise to use a specialized “flip-over plow” that buries seeds like nightshade, purslane and pigweed.

Sprays for weed control must be timely, Hembree said, and growers need to be aware that herbicides can “get tied up” in crop residue. He cited the example of use of Staple after cotton, which left a residue that harmed the following tomatoes.

“That was devastating,” he said.

Literature provided by Shrestha and others also noted that the crop residue can make it more challengng to use pre-emergent herbicides.

Transplants, notably for tomatoes, have a head start on weed pressures, Hembree said. And subsurface drip also lowers weed pressures because there is less moisture at the surface than when flood irrigation is used.

Dan Munk, UC farm advisor for soils and water and cotton for Fresno County, said the research center is changing some of its irrigation practices along with surrounding farmers.

Many of them are moving to subsurface drip irrigation for tomatoes, burying the drip at 8 to 12 inches and using it for six to eight years. One grower told Munk he has used the same drip tape for 17 years.

Munk said drip is less variable in applying water than furrow irrigation. He cited a graph showing that in cotton a furrow system applied more water to one end of a 300 foot bed than another, varying from depths of 4.5 inches to 2.5 inches.

“There is less surface evaporation from subsurface drip, and uniformity with drip is well over 90 percent, compared to 75 or 80 percent with surface irrigation,” he said, adding, “Were also looking backward in time to overhead linear systems and the center pivot.”

Munk and Mitchell conceded there have been particular challenges to getting good cotton yields out of no-till systems with or without cover crops. Munk said some of that has to do with season length in California and weather – such as happened this year – when planting is delayed by cool conditions.

An added challenge: There has been a move by many from Acala to Pima cotton, which takes still longer to mature.

Mitchell said researchers have learned of the need to plant cotton “into moisture.”

“Cotton enjoys a long season, and we try to plant as early as possible,” Munk said.

He said tomato growers are learning they can do deficit irrigation at the end of the season as roots tap deeper into the soil. “In cotton, it’s no different, and it’s a slightly deeper rooting plant,” Munk said. “Some of our best yields are coming off moderately stressed cotton.”